There is a continuing trend in the semiconductor industry to fabricate integrated circuits of increasing complexity. The fabrication of extremely complex, high-density integrated circuits has been made possible through advances in integrated circuit fabrication technology. Fabrication technology now exists having the capability to define circuit components having feature sizes in the sub-micron size range. For example, new lithographic techniques have been developed using X-ray and pulse-laser energy sources. Additionally, film deposition technology now exists with the capability to form thin-films having precisely determined metallurgical compositions and thicknesses. Furthermore, film deposition techniques have been developed which are capable of selectively depositing metals in precisely defined locations during device fabrication.
While advanced fabrication technology can create large numbers of sub-micron sized device components, reducing overall size of a semiconductor device, such as a memory device, is limited because each device component must be electrically contacted by metal leads carrying control signals to and from the device component. However, the number of die on each substrate must by maximized to keep production costs at an economically competitive level. Therefore, the system of metal interconnections to the device components must be configured in such a way as to not increase the overall die size.
To maintain a small die size, high-density integrated circuits are now commonly fabricated with multiple levels of metal interconnects. Typically, the metal interconnect layers are separated by interlevel dielectric layers and electrically coupled by metal-filled vias residing at selected locations in the interlevel dielectric layers. The vias are filled by a metal plug which provides an electrical conduit between overlying metal interconnect layers. Aluminum and aluminum alloys are widely used to form metal interconnects in integrated circuits.
Although aluminum and aluminum alloys can be used to form metallized via plugs, in certain cases aluminum can not be reliably deposited to form a high reliability via plug. Accordingly, tungsten has become an increasingly popular material for the formation of metallized via plugs. Tungsten possesses high electrical conductivity and can be readily deposited into high aspect ratio vias. Additionally, a variety of deposition processes are available for the formation of both blanket tungsten, and selectively deposited tungsten. However, the physical properties of tungsten differ substantially from the physical properties of aluminum and aluminum alloys. Tungsten is a mechanically hard, high-density metal having a high melting point. In contrast, aluminum is a soft, ductile metal having a relatively low melting point, and characterized by large grain structure. When electrons flow from hard metals, such as tungsten, to a softer metal, such as aluminum, a flux divergence occurs at the metallic interface. In the softer metal, the flow of electrons pushes metal atoms in the softer metal away from the interface. This phenomenon is known as electromigration and severely degrades the current handling capability of the metallization structure. Because tungsten has a high density and melting point, it does not experience a mechanical deformation when subjected to a high electrical current. Moreover, tungsten does not self-diffuse when subjected to high electrical current. Therefore, electromigration of the softer metal, in the direction of electron flow, causes a void at the tungsten-aluminum interface.
One method of reducing the flux divergence at the metal interface includes placing a thick, refractory metal barrier between the tungsten plug, and the metal interconnect. However, due to the high electrical resistivity of most refractory barrier materials, the barrier layer must be relatively thick to be effective. Such thick barrier layers add a great deal of complexity and difficulty in the fabrication of the metal interconnect structure. Accordingly, improvements in the structural arrangement of a refractory-metal filled via plug are necessary to overcome the inherent electromigration problems encountered in existing metal interconnect structures.